Oecologia (2013) 172:817–822
DOI 10.1007/s00442-012-2536-0
PLANT-ANIMAL INTERACTIONS - ORIGINAL RESEARCH
Fern and bryophyte endozoochory by slugs
Steffen Boch • Matthias Berlinger • Markus Fischer
Eva Knop • Wolfgang Nentwig • Manfred Türke •
Daniel Prati
•
Received: 25 May 2012 / Accepted: 13 November 2012 / Published online: 1 December 2012
Ó Springer-Verlag Berlin Heidelberg 2012
Abstract Endozoochory plays a prominent role for the
dispersal of seed plants, and dispersal vectors are well
known. However, for taxa such as ferns and bryophytes,
endozoochory has only been suggested anecdotally but
never tested in controlled experiments. We fed fertile
leaflets of three ferns and capsules of four bryophyte species to three slug species. We found that, overall, spores
germinated from slug feces in 57.3 % of all 89 fern and in
51.3 % of all 117 bryophyte samples, showing that the
spores survived gut passage of slugs. Moreover, the number of samples within which spores successfully germinated did not differ among plant species but varied strongly
among slug species. This opens new ecological perspectives suggesting that fern and bryophyte endozoochory by
gastropods is a so-far-overlooked mode of dispersal, which
might increase local population sizes of these taxa by spore
deposition on suitable substrates.
Keywords Dispersal Gastropoda Herbivory Mutualism Spore germination
Communicated by Andreas Prinzing.
S. Boch (&) M. Berlinger M. Fischer D. Prati
Institute of Plant Sciences and Botanical Garden,
University of Bern, Altenbergrain 21, 3013 Bern, Switzerland
e-mail: [email protected]
E. Knop W. Nentwig
Institute of Ecology and Evolution, University of Bern,
Baltzerstrasse 6, 3012 Bern, Switzerland
M. Türke
Department of Ecology and Ecosystem Management,
Technische Universität München,
Hans-Carl-von-Carlowitz-Platz 2 Freising-Weihenstephan,
85350 Munchen, Germany
Introduction
In sessile organisms, such as plants, propagule dispersal is
of particular importance. Many dispersal vectors of plant
propagules are well known and there is a wealth of case
studies on this topic (Schupp et al. 2010). Among dispersal
mechanisms, endozoochory (transport of propagules inside
animals) plays an important role for the dispersal of seed
plants, and might even promote their germination (Beattie
and Culver 1982; Duthie et al. 2006; Schupp et al. 2010).
Similar to many vertebrates, gastropods have a broad diet
spectrum including living and decaying plant material,
fungi, dead animals, and seeds (Speiser 2001; Türke et al.
2012). Gastropod endozoochory has already been shown
for lichens (which are not plants, but symbioses of photobiontic algae or cyanobacteria with fungi), first for the
photobiont component (Fröberg et al. 2001) and recently
also for complete lichens (Boch et al. 2011). However,
whether gastropod endozoochory also occurs with plant
taxa other than seed plants is not known. Cryptogams, such
as ferns and bryophytes, are a very diverse group, and they
inhabit essentially all terrestrial and some aquatic habitats
(Porley and Hodgetts 2005; Kreft et al. 2010). Both ferns
and bryophytes can reproduce sexually by gametangia
(producing sperms and eggs) after spore germination, or
vegetatively by producing propagules such as bulbils or
gemmae, although vegetative reproduction is less common
among ferns (McVeigh 1937; Raghavan 1989; Porley and
Hodgetts 2005). Propagules can be dispersed by wind,
water, or by attaching externally to animals, leading to
exozoochorous dispersal (Kimmerer and Young 1995;
Porley and Hodgetts 2005; Glime 2007). Although ferns
and bryophytes are consumed by various animals, including gastropods (Davidson et al. 1990; Speiser 2001;
Bråthen et al. 2007; Glime 2007; Arosa et al. 2010), there
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are only anecdotal reports that fern and bryophyte spores
(Proctor 1961; van Tooren and During 1988; Davidson
1989; Bråthen et al. 2007; Arosa et al. 2010) and bryophyte
fragments (Parsons et al. 2007) might survive animal gut
passage. However, this has never been systematically tested in controlled experiments. Thus, the potential for endozoochorous spore dispersal of ferns and bryophytes by
slugs has so far largely been overlooked. In addition, it is
not known whether spores of different fern and bryophyte
species differ in their ability to survive the gut passage of
slugs, suggesting adaptation of some species to slug dispersal, as already shown for seed plants (Türke et al. 2012).
Furthermore, it is not known whether slug species differ in
their efficiency to disperse spores. This might, on the one
hand, be the case because of different feeding preferences,
e.g., that generalist species are better dispersers because of
their wider diet spectra, or, on the other hand, by varying
gut conditions allowing spore survival only among slug
species that do not digest spores. In addition, the total
amount of plant material, which is, in the case of ferns or
bryophytes, related to the consumed amount of spores, will
most likely differ among slug species. This might also
affect the efficiency to disperse spores among slug species.
Therefore, we experimentally tested for endozoochorous
spore dispersal by slugs. In particular, we tested whether
(1) fern and bryophyte spores survive and germinate after
passage through slug guts, (2) germination success of
spores differs among fern and bryophyte species, and (3)
slug species differ in how efficiently they disperse fern and
bryophyte species.
Materials and methods
Ferns
Athyrium filix-femina (L.) Roth (Lady fern; Woodsiaceae)
and Dryopteris filix-mas (L.) Schott (Male fern; Dryopteridaceae) are frequent species occurring throughout Central
Europe and growing on various, mainly moderately humid
soils in different forest types. Gymnocarpium robertianum
(Hoffm.) Newman (Limestone oak fern; Woodsiaceae)
occurs mainly in limestone areas throughout Central
Europe in shaded to half-shaded, moderately humid conditions
on scree slopes, in rock crevices, and wall cracks (Nebel
et al. 1993).
Bryophytes
Bryum pallescens Schleich. ex Schwägr. (Tall-clustered
thread-moss; Bryaceae) grows in humid or wet conditions
on rocks and sand. It is widely distributed but locally
rare in Central Europe. Funaria hygrometica Hedw.
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Oecologia (2013) 172:817–822
(Bonfire-moss; Funariaceae) and Leptobryum pyriforme
(Hedw.) Wilson (Golden thread-moss; Bryaceae) are
common, cosmopolitan species with a wide ecological
amplitude growing mainly in open habitats. The liverwort
Pellia endiviifolia (Dicks.) Dumort. (Endive pellia; Pelliaceae) grows on wet calcareous soils or limestone rocks
distributed in limestone areas of the northern hemisphere
(Frahm and Frey 2004).
Slugs
Arion vulgaris Moquin-Tandon (A. lusitanicus Mabille;
Spanish slug; Arionidae) is an introduced species of wide
ecological amplitude and omnivorous feeding behavior. As
a consequence of its high reproductive rate, it is now very
common in gardens and cultural land in Central Europe,
where it has widely replaced the native slug species Arion
rufus (L.) (Red slug; Arionidae). Arion rufus mainly occurs
in forests, but also in open habitats, and has a broad diet
spectrum. Limax cinereoniger Wolf (Ash-grey slug; Limacidae) inhabits different, near-natural forest communities
and mainly feeds on decaying plant material, algae, and
fungi (Turner et al. 1998; Speiser 2001). We chose these
slug species because of their differences in distribution and
ecological demands, and because they are among the
largest gastropods in Central Europe, suggesting that they
might be effective dispersers capable of moving greater
distances than smaller species (Türke et al. 2010).
Collection of the involved species
In early summer 2011, we collected capsules of the four
bryophyte species, fertile leaves of the three fern species,
and 70 A. vulgaris individuals in the botanical garden of
Bern (Switzerland; 46°570 N, 7°260 E). Totals of 70 A. rufus
and 70 L. cinereoniger individuals were all collected at
two forest sites in Thuringia (Germany), dominated by
European beech. These were the Tautenburgerwald near
the city of Jena (50°590 N, 11°410 E) and in the Hainich National
Park near the city of Mühlhausen (51°500 N, 10°270 E).
Feeding experiment and cultivation of bryophytes
and ferns
We performed a factorial experiment feeding ten replicates
of each plant species to individuals of each slug species.
The 210 slugs were kept individually in fauna boxes
(180 9 135 9 65 mm) in a climate chamber (18/16 °C;
16/8 h light/dark cycle; Fig. 1a), where we first fed them
for 72 h with tissue-paper to ensure defecation. Secondly,
ten randomly selected individuals of each of the three slug
species were fed for 48 h with fern and bryophyte material.
Each slug individual was fed with either fertile leaflets of
Oecologia (2013) 172:817–822
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Fig. 1 a The feeding experiment showing the 210 fauna boxes with
slugs in a climate chamber. b–h Microscope images showing
prothallia of b A. filix-femina, c D. filix-mas, d G. robertianum, and
protonema of e B. pallescens, f F. hygrometica, g L. pyriforme, and
h P. endiviifolia germinated from slug feces
each of the three fern species or with approximately 30
capsules of each of the four bryophyte species. As a proxy
for the consumed amount of spores, we roughly estimated
the consumed amount of fern and bryophyte material using
an ordinal scale (1: less than a third, 2: one- to two-thirds,
3: more than two-thirds). Then, we cleaned the slugs under
running water to avoid secondary contamination with
spores and transferred them into new fauna boxes with
tissue-paper for defecation. After 48 h, we collected all
fecal pellets of each slug individual from the fresh boxes to
exclude contact of fecal pellets tested for spore germination
with plant material. Fecal pellets were put in one separate
Petri dish per slug individual onto phytagel (Sigma, USA),
a culture medium without nutrients. Thus, each Petri dish
represented one sample. From 28 June to 9 August 2011, we
incubated fecal pellets in a climate cabinet (15 °C; 16/8 h
light/dark cycle). Finally, we inspected all fecal pellets with
a dissecting microscope and recorded germination of spores
for each Petri dish; germination was visible as growth of
bryophyte protonema or fern prothallia.
Table 1 Results of a GLM analysis, testing for differences in the
number of samples in which spores germinated successfully among
the seven plant and three slug species
Statistical analysis
We analyzed numbers of samples with successful spore
germination among plant species and among slug species
with a generalized linear model. We also included the
consumed amount of plant material as a covariate and the
interaction between plant and slug species. Data were analyzed using R v.2.13.1 (R Development Core Team 2011).
Results
Overall, spores germinated from slug feces in 57.3 % of all
89 fern and in 51.3 % of all 117 bryophyte samples.
Number of samples with germination
success
df
Deviance
Consumed plant material
2
9.415
Plant species
6
11.409
0.077
Slug species
2
76.698
\0.001
0.067
Plant species 9 slug species
12
20.002
Residuals
183
166.81
p
0.009
On average, spores germinated in 60.0 % of samples of
A. filix-femina (n = 30; Fig. 1b), 62.1 % of D. filix-mas
(n = 29; Fig. 1c), 50.0 % of G. robertianum (n = 30;
Fig. 1d), 44.8 % of B. pallescens (n = 29; Fig. 1e), 55.1 %
of F. hygrometica (n = 29; Fig. 1f), 51.7 % of L. pyriforme (n = 29; Fig. 1g), and 53.3 % of P. endiviifolia
(n = 30; Fig. 1h). These findings indicate that fern and
bryophyte spores passed the slug guts without being
digested and developed into juvenile bryophytes and ferns.
The number of samples in which spores successfully
germinated did not differ among plant species but varied
strongly among slug species (Table 1; Fig. 2), with significantly lower mean values in L. cinereoniger samples
(12.9 %) than in A. vulgaris (75.7 %) and A. rufus
(74.2 %) samples. Limax cinereoniger did consume less
plant material (in 97 % of all cases they consumed less
than one-third of the material available) than A. vulgaris
(56 %) and A. rufus (80 %). However, this does not seem
to be the only reason for the observed differences among
slug species because the slug species effect remained significant after correcting for the amount of plant material
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820
Fig. 2 The effect of plant and
slug species on the percentage
of samples in which spores
successfully germinated
(means ? SEM)
Oecologia (2013) 172:817–822
Plant species
Slug species
Arion rufus
Funaria hygrometica
Arion vulgaris
Leptobryum pyriforme
Limax cinereoniger
Pellia endiviifolia
Bryum pallescens
Athyrium filix-femina
Dryopteris filix-mas
Gymnocarpium robertianum
0
20
40
60
80
100
Percentage of samples in which spores germinated
consumed. In addition, the germination success in L. cinereoniger samples was still significantly lower than in
samples of both Arion species. The results did not change
qualitatively when analyzing the germination success of
bryophyte and fern spores separately, except that the
marginally significant plant species–slug species interaction became significant in the bryophyte-only dataset
(p = 0.016; Fig. 2). This interacting effect reflects that the
germination success of B. pallescens did not differ between
L. cinereoniger and the Arion species, in contrast to the one
of the other bryophytes.
Discussion
Gastropods are important herbivores of seed plants (Speiser
2001). Although they are also important consumers of
bryophytes and ferns, this has rarely been documented
(Davidson et al. 1990; Speiser 2001; Glime 2007), and
endozoochorous dispersal of fern and bryophyte spores has
only been suggested anecdotally and never tested in controlled experiments. For instance, Davidson (1989) reported better germination of the spores of two moss species,
Brachythecium rutabulum (Hedw.) Bruch, Schimp &
W.Gümbel and Mnium hornum Hedw., after gut passage
through two Arion species. Furthermore, Proctor (1961) fed
samples of the liverwort Riella americana M.Howe &
Underw. to three domesticated mallard ducks and showed
that liverwort spores can survive gut passage and germinate
from duck feces. Fern and moss growth has also been
observed from the feces of reindeer (Bråthen et al. 2007),
earthworms (van Tooren and During 1988), and flying
foxes (Parsons et al. 2007). Based on fern and moss growth
from incubated feces samples, the authors proposed endozoochorous dispersal but did not test for it experimentally.
In our controlled experiment, the spores of three fern and
four bryophyte species survived passage through the guts
123
of three slug species, without being digested, and germinated from slug feces. These results demonstrate that
endozoochorous fern and bryophyte dispersal is possible.
The importance of fern and bryophyte endozoochory,
relative to dispersal by water or wind, remains unknown.
Endozoochorous spore dispersal by slugs is probably less
important than wind for long-distance dispersal but might
be more important for short-distance dispersal. Nevertheless, Türke et al. (2010) showed that A. rufus may endozoochorously disperse plant seeds up to at least 15 m.
This is much farther than the dispersal distances measured
by Kimmerer and Young (1995) for asexual brood branches of the moss Dicranum flagellare Hedw., which were
dispersed externally on Philomycid and Arionid slugs
(max. 23 cm, mean 3.7 cm). However, long-distance dispersal of fern and bryophyte spores by gastropods might be
possible in some cases, as gastropods themselves can be
dispersed by wind or animals (Rees 1965; Gittenberger
et al. 2006; Wada et al. 2011).
In addition to dispersal, successful plant establishment
requires firstly arrival at an appropriate site, and secondly
nutrients to grow. We suggest that endozoochory by slugs
may ensure both requirements: slug feces are sticky and
could therefore attach spores to an appropriate substrate,
and feces may even promote growth as they are rich in
nutrients. Kimmerer and Young (1995) reported increased
substrate adherence of moss fragments by slug secretions,
in line with our suggestions for spores. Our results imply
that, despite losses due to gastropod herbivory, local population sizes of bryophytes and ferns might be increased by
endozoochorous spore dispersal, suggesting a mutualistic
relationship between gastropods and these cryptogams.
In our study, the number of samples in which spores
successfully germinated did not differ among plant species,
despite differences between our experimental fern and
bryophyte species in their ecological amplitudes and range
sizes. This implies that endozoochorous dispersal of spores
Oecologia (2013) 172:817–822
by slugs might be a general phenomenon among ferns and
bryophytes. However, it remains open whether spore germination after slugs’ gut passage would differ among fern
and bryophyte species with even more pronounced differences in their ecological characteristics than among the
plants we used in our experiment. This would suggest that
the importance of endozoochorous spore dispersal by slugs
might differ among habitats.
We found that fern and bryophyte germination success
differed strongly depending on the slug species, with
generally higher germination success for ferns and bryophytes regenerating from the feces of Arion species than
for those regenerating after passage through L. cinereoniger. This may well be due to the wider diet spectra of the
Arion species compared with L. cinereoniger (Turner et al.
1998; Türke et al. 2010). In our experiment, this was
reflected in the generally lower amounts of plant material
consumed by L. cinereoniger. However, the slug species
effect was still present after correcting for the consumed
amount of plant material in our analysis. The number of
spores consumed may still differ among slug species,
explaining part of the variability among them, but unfortunately we could not quantify the exact number of consumed
spores. Thus, it remains unclear which slug species-specific
factors, such as microbial or biochemical differences in gut
microenvironment (Charrier and Brune 2003), affect the
endozoochorous dispersal of spores.
Conclusions
We showed experimentally that fern and bryophyte spores
can survive slug guts and germinate from feces, which so
far had been largely overlooked. Given that gastropod
herbivory on these taxa commonly occurs in nature, our
findings imply that endozoochory of fern and bryophyte
spores by gastropods is a so-far-overlooked mode of
dispersal, which might increase local population sizes by
spore deposition on suitable substrates.
Acknowledgments We thank A. Bergamini for the identification of
Bryum pallescens, S. Braybrook, K. Esfeld, and T. Imhof for assistance in the laboratory, A. Gygax for preparing Fig. 1, and E. Allan
for revising the English.
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